As is well known, electrical brushes are merely devices for conducting current to or from a rotating part. Typically, the brush is stationary, and is held and guided by a fixed brush holder in which it slides freely. There may be several brushes side by side to form a single brush set. The rotating member may be the commutator of a direct current machine or the slip rings or the like of an alternating current machine, but in general brushes are usable in any instance where current is to be passed from one member to a relatively movable member. A typical brush material is carbon, to which circuit connections are made by means of short flexible cables. Brushes wear and must be replaced periodically, and it is known in the art to provide brush wear sensors, typically in the form of an electrical wire or the like incorporated in but insulated from the brush body, and so located as to activate a sensor or signal when the insulation is worn away when brush wear reaches a predetermined point.
A typical previous approach has been to use a length of standard Teflon (trademark) coated wire, cut square at one end and sealed with an insulating cement to insulate the exposed wire tip. This wire is then cemented into a blind hole, typically slightly deeper than, and parallel to, the main conductors extending into the brush body. When the brush wears to the wire tip, and the wire tip sealant wears away, the circuit to a signal is completed. Difficulties have been encountered in anchoring such single wires because of the nature of the required high temperature insulating material, typically being polymers of fluoroethylene, having a smooth, generally inert surface that is difficult to bond in the brush body. Additionally, there has been the problem of insulating the cut end of the wire to insulate this cut end from the brush. Silicone sealants have been tried and have been successful to some extent, but silicone can be detrimental to brush performance. Additionally, there is the possibility of faulty application of the insulating sealant, which will result in a false signal. Additionally, creep of the original insulation adjacent its cut end can result in exposure of the conducting wire to the brush material.
In general, the concept of the instant invention utilizes the normal wire insulation itself to insulate the wire from the brush body, such that there are no insulation discontinuities within the brush body, and there is no need to apply a special insulator material to any part of the wire in the brush body. Basically, the invention utilizes an insulated wire doubled back onto itself intermediate its ends and located in a passageway such as a blind bore in the brush body such that the doubled back portion extends to the point at which brush wear is to be sensed and the ends extend from the brush body for connection to one or more electrical signal or sensor devices. Thus, the normal wire insulation is continuous within the brush body. The doubled wire length within the brush body is secured in position by a suitable cement. To minimize the possibility of inadvertent pullout, it is preferred to twist the individual wire segments of the doubled wire about each other, since after the cement adheres to the brush material and sets, the wire is additionally mechanically held. This precludes failure due to nonadherence of the cement to the insulation on the wire.
It might be expected that brushes would be replaced promptly after the wear indicator signal is activated, such that a continuous signal thereafter would not be necessary. However, it must be assumed that brushes will not be replaced immediately, and cases can be expected where replacement does not occur for some time. Therefore, it is preferred that the sensor wire be cemented over substantially the full depth of the blind pole, such that the wires would remain in the hole and continue to provide a wear signal until the brush is actually replaced.
In those wear sensors utilizing a single wire, there is the possibility that the insulation might pull across the wire end while being worn away, in which case there would not be a signal since there would be no contact at times. This is also true of the doubled wire approach to some extent, but the double wire in essence provides to opportunities at making contact. As such, the double wire can have both ends connected to one terminal, or the two ends could lead to different functional applications, such as having one end lead to a wear indicator light and the second end lead to a lift-off timer circuit.
Other features and advantages of the invention will become apparent to those skilled in the art from the ensuing description of preferred embodiments, taken in conjunction with the appended drawings.